IMPACT OF THERMAL STRESS AND HIGH VPD ON GAS EXCHANGE AND
CHLOROPHYLL FLUORESCENCE OF CITRUS GRANDIS UNDER DESERT
Development Yotvata Faculty of Agricultural, Food and
Additional index words: photosynthesis, photoinhibition, heat stress, temperature Abstract The photosynthetic response of Citrus grandis
to high light intensities, low air humidity and high temperature stress was investigated under desert conditions in the southern AravaValley (Israel). During summer, a typical midday stomatal closure was observed even in well-watered trees due to the dramatic increase of the leaf-to-air water vapour deficit. As a result of the reduced transpirational cooling, leaf temperatures increased up to 11 °C above ambient air temperature. The combination of heat stress and photoinhibition resulted in a reversible decrease of photosynthetic activity of Citrus grandis
under the extreme summer conditions. 1. Introduction
, native of subtropical and tropical regions in Eastern Asia became widely
cultivated in areas with Mediterranean climate. In Israel, Citrus grandis
, C. sinensis
, C. limon
and C. paradisi
are major fruit crops of the coastal plain, where the mean annual rainfall ranges between 500 and 700 mm (Bielorari et al
., 1973) and average temperature at midday in the August is 30 °C (Cohen et al.
During recent years cultivation of Citrus
has substantially increased under desert
conditions in the Negev and in the Arava Valley of Israel. In these arid areas crop production requires intensive irrigation. The climate of the Arava is characterized by mild winters and very hot summers with high air temperatures (> 42 °C), low air humidity and high radiation. Optimum temperature for Citrus
photosynthesis varies from 25°C to 30°C, while temperatures above 35°C reduce photosynthetic activity (Spiegel & Goldschmidt, 1996). High temperatures combined with low air humidity may induce stomatal closure. In addition the combination of high irradation and stress conditions, which limit photosynthetic energy conversion by reducing CO2 supply, may result in enhanced photoinhibition (Baker, 1993: Blanke, 2000; Herppich 2000). Photosystem II is that component of the photosynthetic apparatus which is most sensitive to heat (Bilger et al
., 1987) and high light stress (Baker, 1993). In the present study, the response of transpiration, stomatal opening, CO2 uptake and chlorophyll fluorescence of Citrus grandis
was investigated under extreme desert conditions. 2. Materials and methods
Three year old Citrus grandis
trees were investigated during June and December 1997
at the Arava Research Station in Yotvata, Israel (29° 53´ N, 53° 3´E ), 40 km north of Eilat. The orchard was irrigated automatically by a drip system every second day during summer and every fourth day during winter. The soil was an Arava loamy sand.
Proc. 2nd Conf. (Sub)trop. Fruits Eds M. Blanke & J. Pohlan
Net CO2 exchange and transpiration of fully expanded leaves were measured with a portable CO2/H2O porometer system (HCM-1000, Walz GmbH, Effeltrich, Germany) under ambient conditions according to Veste & Herppich (1995) and von Willert et al. (1995). Ambient CO2 concentration was nearly constant at 350 ppm throughout the day. Area (projected leaf area) based gas exchange parameters were calculated after von Caemmerer and Farquhar (1981). Simultaneously with gas exchange, chlorophyll fluorescence was monitored on the same plant with a Mini-PAM fluorometer (H. Walz GmbH, Effeltrich, Germany). Maximum photochemical efficiency of PSII (Fv/Fm = Fm-F0/Fm) was determined on dark-adapted (10 to 12 min) attached leaves (n = 7). Fm, Fv and F0 denote the maximum, the variable and the initial fluorescence, respectively (cf. von Willert et al
. 1995). Quantum yield of linear electron transport (F´m- F / F´m = ΦPSII) were analysed as summarized by von Willert et al
. (1995) and electron transport rate (Je) was calculated according to Krall & Edwards (1992) as Je = ΦPSII * 0.5 * 0.84 * PPFD (PPFD = photosynthetic photon flux density). 3. Results Typical diurnal time courses of net CO2 exchange (JCO2), transpiration (JH2O), leaf conductance (gH2O) of Citrus grandis
and the microclimatic conditions on a normal summer and winter day are shown in Fig. 1. Maximum net CO2 exchange rates of C. grandis
typically ranged between 8 and 12 µmol m-2 s-1 throughout the year. Both photosynthetic electron transport (Je, Fig. 2) and net CO2 exchange (data not shown) were always light saturated at PPFD above 800 µmol m-2 s-
1. During winter month diurnal courses of leaf conductance, net CO2 exchange and transpiration were very simimilar, and no midday depression of gas exchange occured (Fig. 1). Maximum air temperatures on cloudless winter days reached 20 to 22 °C and mean maximum ∆w did not exceed 20 kPa MPa-1.
Figure 1: Typical diurnal courses of net CO2 exchange (JCO2, A), transpiration (JH2O, B), leaf
conductance (gH2O, C), photon flux density (PPFD, D), leaf and air temperature (Tair, Tleaf, E) and air-to-leaf water vapour deficit (∆w, F) in June (circles) and December (solid line).
Figure 2: Light response of photosynthetic electron transport (Je) of Citrus grandis
In contrast, mean maximum air temperatures varied from 33 to 37 °C during summer days. The temperature of sun-exposed leaves raised up to 11 °C above air temperature during hours of high radiation. As a result, ∆w increased to more than 85 kPa MPa-1 (Fig. 1). Due to the harsh substantially decreased after ca 9:00 even in well-watered plants.
Midday stomatal closure reduced net CO2 uptake by nearly 65%. Additionally, high
temperature in combination with high irradiance directly affected photosynthesis (Fig. 4). This was indicated by the increase of internal CO2 concentration around noon (Fig. 4B). Furthermore, maximum photochemical efficiency (Fv/Fm) decreased by 12% from pre-dawn to noon (Fig. 4C). The initial fluorescence (F0) was nearly unaffected in the early morning hours and tended to rise slightly when leaf temperature exceeded 27°C. At leaf temperatures of 43°C the initial fluorescence was 15% higher than pre-dawn levels (Fig. 4C).
Figure 3: Plot of leaf conductance for water vapour over air-to-leaf water vapour deficit. Given
are the results of four summer days (June).
Air temperatures occasionally exceeded 40 °C. On these occassions leaf temperatures
of sun-exposed leaves increased up to 47.5 °C. Under these conditions, JCO2 was seriously reduced by the pronounced stomatal closure. Interestingly, leaf conductance at maximum ∆w (max. Tleaf = 47.6 °C) was higher on that hot day than on cooler days (max. Tleaf < 43 °C). Maximum transpiration rate increased from 3.57 to 5.9 mmol m-2 s-1.
This higher transpiration rate significantly contributed to the cooling of the leaf and might have prevented the increase of leaf temperature above a critical point. Heat stress above leaf temperatures of 45°C lead to a decrease in the photosynthetic activity (Fig. 4). Nevertheless, in these heat stressed leaves midday internal CO2 concentration (ci) was nearly 50 ppm higher (ci = 253 ppm) as compared to leaves experiencing normal summer temperatures (Fig. 4B) and Fv/Fm declined to a lower level (Fig. 4D). The mean initial fluorescence rose by 25% from 436 to 537 when leaf temperatures increased from 45°C to 47.6 °C indicating serious heat stress (Fig 4C). Both F0 and Fv/Fm recovered during the afternoon when leaf temperature and radiation declined again (Fig. 4D).
Figure 4: Diurnal courses of leaf temperature (Tleaf, A), internal CO2 concentration (ci, B),
initial fluoresecence (Fo, C) and maximum quantum yield of PS II (Fv/Fm, D) on an normal summer day (squares) and under heat stress (circles).
Maximum net CO2 exchange rates of Citrus grandis
observed in this study under
extreme desert conditions corresponded well with values (4 to12 µmol m-2 s-1) obtained for for Citrus sinesis
and Citrus paradisci
under moderate environmental conditions (Singlair & Allen, 1982; Spiegel & Goldschmidt, 1996; Blanke, 2000). Photosynthesis of citrus was saturated at relatively low light intensities around 800 µmol m-2 s-1 in this study. Low light saturation between 600 to 800 µmol m-2 s-1 are well known for other Citrus
species (Singlair & Allen, 1982; Syvertsen, 1984; Braake & Allen, 1995). This indicates that sun-exposed leaves are exposed to an excess of light during most of sunny cloudless days. This easily explains why maximum CO2 exchange rates did not differ between summer and winter as PPFD were always above the point of light saturation.
A negative influence of ∆w on the stomatal opening was observed also in this study
for well-irrigated citrus trees in the dry summertime as found for other Citrus
cultivars (Hall et al
., 1975; Kriedemann & Baars, 1981; Braake & Allen, 1995). A midday depression of stomata opening is typical for other well-irrigated fruit trees such as apricots (Prunus armeniaca
) and grapes (Vitis vinifera
) (Lange & Meyer, 1979). Syvertsen (1982) hypothesized that the direct effect of ∆w on leaf conductance may explain similar transpiration rates under different environmental conditions. However in
our case, leaf conductance in Citrus grandis
was similar in winter and summer and transpiration rates were even higher in summer. Thus, it may be assumed that Citrus
has to reduce the maximum transpiration rates because water uptake and transportation capacity are limited by a low hydraulic conductance (Kriedemann & Baars, 1981; Moreshet et al
., 1990). The fact that the midday stomatal closure in the fruit trees is controlled by ∆w has an important implication on the irrigation management. Increasing the soil water content by applying a greater irrigation volume will not result in higher leaf conductance and higher CO2 uptake midday (Lange & Meyer, 1979). As a result of the low transpiration rates leaf cooling was reduced and leaf temperatures exceeded air temperatures up to 11 °C. This overheating of the leaves caused a drastic increase of ∆w, thus negatively affecting leaf conductance and CO2 uptake. If CO2 uptake was only restricted by stomatal closure than the internal CO2 concentration should decrease. However, under those high temperatures observed in our study ci significantly increased. This clearly indicates photoinhibition and/or other down-regulating systems (Clifford et al
., 1997; Herppich et al
., 1997). High photon flux density may induce photoinhibition by overexcitation of photosystem II when stomatal closure limited CO2 supply (Baker 1993). Furthermore, photorespiration may increase under high light intensities and high temperatures thus contributing to the increase of ci. In the investigation presented, PSII was not much affected in comparision to the investigations of Werner et al
. (1999) and Herppich (2000). Fv/Fm declined only by 12% even at leaf temperatures above 42 °C. In addition, predawn values of Fv/Fm were relatively high if compared with other evergreen Mediterranean trees (Werner et al
., 1999). Photorespiration provides an effective electron sink and may help prevent excessive reduction of electron transport chain and photoinactivation of electron transport during light stress (Heber et al
An increase of photorespiration may explain the low reduction of the maximum
photochemical efficiency of the photosystem II in Citrus
leaves. Heber et al
. (1996) showed that photorespiration is a useful physiological way to prevent photoinhibition under stomatal closure.
The dramatic decline of Fv/Fm and the rise in F0 which was observed at temperatures
above 47°C indicates the inhibition of the photosynthetic apparatus by heat stress (Bilger et. al. 1987, Yamane et al
., 1997; Herppich et al
., 1994). However, as described for other mediterranean trees (Bilger et al
., 1987), serious heat stress can be observed above 45°C. In our study, any inhibition was reversible and Fv/Fm increased to predawn values as leaf temperature decreased during the afternoon. Short term experiments indicated that acclimatization to heat stress may occur. Weis & Berry (1988) showed for bean leaves that a 3-hour pre-adaptation at 40°C caused an increase in the tolerance limit by about 4°C. In potatoes a moderate increase of temperatures in 2.5 hours from 25°C to 38°C caused rapid photosynthetic acclimatization and increased the critical temperature from 38°C to 43°C (Havaux, 1993). Also, growth temperatures may largely influence temperature optimum photosynthesis (Björkman et al
., 1980). Under field conditions Citrus
will be able to adapt to increasing temperature during the growing season. Desert evergreen species, which experience large seasonal changes in temperature show large changes in their thermal stability. The shift in thermal tolerance can be up to 10°C and even higher values have been reported (Berry & Björkmann, 1980). Our experiments show for citrus that PSII acclimates to heat stress during the season. Thus, Citrus grandis
seems to be well adapted to the high photon flux density and heat stress occasionally occuring in the Arava Valley. Acknowledgments We thank Werner Herppich (Potsdam), Yoav Waisel (Tel Aviv) and Christiane Werner (Bielefeld) for their helpful comments. This research was funded by the German Federal Ministry for Education and Research (BMBF, Projekt 0339676).
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Dr Dominic Heaney MA MRCP PhD ______________________________________________________________________________ Curriculum Vitae Dr Dominic Heaney MA (Cantab) MB BCh (Oxon) Consultant Neurologist and Honorary Senior LecturerNational Hospital for Neurology and Neurosurgery Dr Dominic Heaney MA MRCP PhD ______________________________________________________________________________ Con
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